Conventionally, a compound semiconductor such as GaN has been known. A method for producing a substrate of such a compound semiconductor using a wire saw has been known (for example, see Japanese Patent No. 2842307 (Patent Document 1) and Japanese Patent Laying-Open No. 2006-190909 (Patent Document 2)). A substrate obtained by cutting using such a wire saw has a surface with a damaged layer. Hence, a main surface (cut surface) of the substrate is etched, grinded, polished, and mechano-chemically polished.
Further, the substrate formed of the compound semiconductor such as GaN is required to achieve suppressed warpage and improved surface flatness, in order to improve properties of an epitaxial layer grown and formed on the main surface of the substrate. In order to improve the properties in shape such as the surface flatness of the substrate, for example, Japanese Patent Laying-Open No. 2004-356609 (Patent Document 3) has proposed to use not only abrasive grains but also a predetermined chemical solution for the GaN substrate so as to perform chemical machinery polishing (CMP). Further, Japanese Patent Laying-Open No. 2005-136167 (Patent Document 4) has proposed to partially removing, by means of etching, a damaged layer formed by grinding and polishing the main surface of the GaN substrate, so as to control stress resulting from the damaged layer. Accordingly, warpage of the substrate is suppressed.